Response of soil nitrogen fractions and nitrogen transformation related enzyme activities to different forms of nitrogen input in salinized grassland of Hexi

The impact of the atmospheric nitrogen deposition on the cycling of soil nitrogen in ecosystems is growing amidst the backdrop of global climate change. The effect of nitrogen deposition on soil nitrogen is dependent on differences in soil nitrogen convertase activity. To investigate changes in the characteristics of soil nitrogen and the factors influencing the nitrogen invertase activity of nitrogen deposition, three different forms of exogenous nitrogen input treatments were established in salinized grassland based on a field experiment in the Hexi Corridor, China: organic nitrogen (ON), inorganic nitrogen (IN), and mixed nitrogen (MN), in addition to a control (CK). The input of pure nitrogen was 15 kg·(hm² ·a)⁻¹. The effects of different nitrogen inputs on the enzymatic activity of soil nitrogen conversion were analyzed. The results showed that nitrogen input of different forms decreased nitrite reductase activity compared with CK, while the input of mixed nitrogen and inorganic nitrogen significantly increased the nitrate reductase activity by 20.2% and 8.1%, respectively. Furthemore, the input of inorganic nitrogen significantly increased the urease activity (16.0%). The addition of various forms of nitrogen resulted in alterations to the soil’s inorganic nitrogen content, with ammonium nitrogen (NH₄⁺-N) exhibiting a more pronounced promotional effect compared to nitrate nitrogen (NO₃⁻-N). MN treatment significantly increased the total nitrogen (TN), NH₄⁺-N, and NO₃⁻-N content in the soil. Taken together, the results suggest that mixed nitrogen input enhances the soil nitrogen content and enzyme activity in salinized grassland. Further studies will be needed to determine the influence of nitrogen deposition on soil nitrogen dynamics in salinized grassland over time. These findings offer a scientific foundation for evaluating and forecasting the soil nitrogen cycle in salinized grassland ecosystems in the modern-day context of global climate change.